Column for electron microscopes



Se t. 22, 1964 A. P. WILSKA COLUMN FOR ELECTRON MICROSCOPES Filed July 5, 1962 INVENTOR. A um? P M L SKA Afro/Pairs nsen United States Patent Oil ice 3,15%,255 Patented Sept. 22, 1964 3,159,255 E -13R ELECRDIJ MI CRGSQQPES '-Jns a, Tucson, Arie, assignor to P Elec- This invention relates to electron microscopes and parularly to the column through which the electron beam the microscope passes in going from the source to the mags plane.

The column of an electron microscope is the heart of the instrument for it is there that the electrons from the source are formed into a beam to impinge upon the object under study and it is also there that the beam is focused and the electron image magnified for presentation on the image plane, which is usually either a fluorescent screen or a photographic f lm. The quality of the image formed is directly controlled by the forces which afiect the electron beam in passing through the column. Some of these forces are the forces produced by focusing ields of the electron lenses and hence are desirable, but other forces are due to stray fields, and these latter forces have a deleterious effect upon the quality of the beam and of the image. One source of such stray fields is the accumulation of electrically charged particles of dust and the like that are attracted to and adhere to the interior wall of the column. Another source is a thin layer or" oil,

:hich comes from the vacuum pumps and forms on the inner walls of the column.

The present invention minimizes the effect of stray electric fields by dividing the most sensitive parts of the column of the microscope into a plurality of hollow metal cylinders placed end to end and each having a frustoconical bore so oriented that its constricted end faces the electron beam source. Furthermore, the included angle of the bore of each of these metal cylinders is large enough so that the electrons can, at most, strike only the constricted edge thereof and cannot hit along the major portion of the length of the inner surface of each of these cylinders.

The invention will be discussed in greater detail in connnection with the drawing, in which the only figure is a side view of an electron microscope constructed according to the invention with parts of the microscope shown in cross ection to illustrate the consL- uction of the column.

The microscope, which is generally indicated by reference character 11, comprises at one end an electron source 12 which may be of conventional nature. Electrons from the source 12 emerge along an axis 13 under the force of attraction of an anode 14 having a central aperture 16. The source 12 is connected to the negative terminal of a power supply which has a positive terminal connected to the anode 14. By virtue of the features of the microscope described herein, 1 have been able to obtain excellent electron images with a power supply voltage of about 5,800 volts to about 50,000 volts with greatly increased contrast ratios over previous microscopes. The electrons are first acted upon by a condensing lens held in the region 17. This condensing lens is created by a mangetic field produced by a coi of wire 13 and is constricted by means of a ferromagnetic structure consisting of an annular disc 19 terminating at its outer periphery in a cylindrical wall 21 and at its inner perimeter in a small cylinder 22. Both of these cylinders are of ferromagnetic material and the latter is provided with a somewhat conicallvshaped, inwardly extending apertured end 23. A second and thicker annular disc 24 of ferromagnetic material provides a low reluctance magnetic path from the end of the cylinder 21 remote from the disc 19 in to a ferromagnetic disc 26 having a central aperture which, together with the aperture of the end member 23, shapes the magnetic field of the condensing lens.

The annular disc 24 also serves as part of the holding means for an obiect to be studied. This object is placed in a support member 27 so as to be struck by electrons of the earn traveling along the axis 13.

Immediately aiter striking the obiect in its holder 27, the electron passes through a focusing lens commonly known as an objective lens in the region 28. This obiective lens is formed by the magnetic field of a coil 29 and is constricted by ferromagnetic members surrounding the coil 29, including the annular disc 24 and a cylindrical extension 31 thereof. A smaller apertured, ferromagnetic disc 32 fits witnal he disc 24 and provides one of the inwardly directed edges that helps define the shape of the magnetic field in the critical region 28. Another ferromagnetic annular disc 33 provides a low reluctance, magnetic path from the other end of the cylinder 31 in to a hollow ferromagnetic member 1%. The latter memher is provided with a generall conically-shaped end 36 having a central aperture therein defined by inwardly extending edges surrounding the axis 13. These edges cooperate with the inwardly extending edges of the disc 32 to define the shape of the magnetic field in the lens region 23.

The inner member 3- is one of the components of the microscope that defines the inner column of the instrument. It is wi hin this column that stray particles of dust frequently come to rest after having been introduced into the instrument when specimens are replaced in the holder 27. Dust particles also enter when the microscope is dismantled to replace components that wear out.

In accordance with this invention, the innermost members of the column are in the form of a plurality of hollow, ferromagne ic cylinders 37, 38, and 39 which are spaced along the column and which are coaxim with the axis 13 of the electron beam. Furthermore, each of the cylinders 37 to 39 has a conical inner here with an included angle determined by the distance of the cylinder from the center of the objective lens location 23. The center of this lens may be considered to be the point of origin of electrons in the beam, and these electrons travel, in general, along straight lines in passing through the cylinders 37 to 39. The angle of each of the cylinders is such that its apex lies between the central region 28 of the objective lens and the proximal end of the cylinder in question. Thus, the included angle of the inner bore of the cylinder 37, which is closest to the region 28, is larger than the include-d angle of the second cylinder 3%, which, in turn, is larger than the included angle of the third cylinder 39. The conical end 36 of the hollow member 3% also fo lows the same law.

The delicate nature of the electron beam in the region of the objective lens is illustrated by the care taken to produce a precisely correct configuration of the magnetic field in the region 28. In addition to using care in construction of the components just described, it has been found desirable to provide a number of stigmator coils to produce correcting fields in the region 28. In one embodiment of the invention I have provided eight such coils, of which only two are indicated in the drawing. These two are designated by reference characters 41 and and comprise several hundred turns of exceedingly fine wire wound on a mandrel. Each coil is provided with a slender ferromagnetic core, that of coil 41 being indicated by reference character 43 and that of coil 42 being indicated by reference character 44-. These stigmator coils are securely held within a non-magnetic structure 46.

A magnifying electron lens is formed by the field of yet another coil 47. The magnetic field of the coil is shaped, or constricted, by ferromagnetic members including the'annular disc 33 and a cylindrical extension 48 of the periphery thereof. An inner hollow cylinder 49 extends from the inner perimeter of the disc 3?: and terminates 'in' an inwardly extending end 51 having a central aperture through which the axis 13 passes. The cylinder 49 actually forms a magnetic extension of the cylinder 34 and in order to improve magnetic shielding, a non-magnetic hollow cylinder 52 is provided to fit into the cylinders 34 and 49 and around the cyhnders 37 to 39. The latterare therefore magnetically connected to the cylinders 34 and 49 only near the ends thereof. The remaining magnetic members surrounding the coil 47 consist of an annular disc 53 magnetically connected to the end of the cylinder 48 and terminating in a hollow ferromagnetic cylinder 54 which has a somewhat conical apertured end 56 facing the end 51 of the cylinder 49 and cooperating therewith to shape the magnetic field in thelens region 57. The cylinders 49 and 54 are spaced apart by a hollow non-magnetic spacer 58.

The final lens in the microscope is a projection lens formed by another coil 59 surrounded by ferromagnetic members comprising the annular disc 53, a cylindrical extension 61 of the periphery thereof, another annular disc 62 at the other end of the cylinder 61 and three inner ferromagnetic cylinders along the center of the column. The first of these cylinders is indicated by reference character 63 and is provided with a conical inner bore, the included angle of which is determined by the same law that governs the anglesof the cylinders 37 to 39 except that the apex of the angle of the bore of the cylinder 63 lies between the end of that cylinder and the center of the lens region 57 which is nearest to it. The other two ferromagnetic members are cylinders 64 and 6% with facing, somewhat conical, apertured ends 67 and 68, respectively, which define the projection lens field in the region 69. These cylinders are spaced apart by a hollow,

non-magnetic cylinder 71.

' Beyond the projection is a large chamber 72 within which may be placed a photographic film holder. At the end of the chamber 72 directly on'the axis 13 is a viewing window 73 which is described in my application entitled Viewing Window for Electron-Optical Devices, Serial No. 207,672 nee concurrently herewith. The window has a concave inner surface 74 in which a thin layer 76 'of fluorescent material is placed to transform the electron beam image into a light image. The window is held in place by a clamping ring 77 and is sealed by gaskets 78-89.

In assembling the microscope the various cylinders 21, 31, 48 and 61 with their appropriate coils are stacked up and aligned by matching keys and keyways in the cylinders and their respective annular discs. The central partof the column is hermetically sealed by O-ring gaskets 81 at the various joints so that the spaces in which the lens coils 18, 29, 47, and 59 are located need not'be evacuated.

The inner cylinders between cylinder 34 and cylinder 66 may be inserted after the outer parts of the microscope, since it is to be noted that there is a uniform inner bore into which the inner cylinders fit. They are then held in place by an externally threaded nut 82.

The whole assembly is aligned and clamped together by one or more bolts 83 and by a hollow pipe 84 which connects the chamber 72 to an evacuating channel 86.

The latter is also connected to a channel 87 directly into the base region of-the instrument. This permits rapid evacuation of the instrument from both ends of the column.

While this invention has been described in terms of a specific embodiment, it will be readily understood that modifications'may be made in it within the ambit of the following claims without departing from the true Sco e of the invention. a at is claimed is:

1. An electron microscope comprising: a source producing a beam of electrons'traveling along an axis; an electromagnetic lens structure comprising a coil of wire substantially coaxial with said beam and a hollow toroidal ferromagnetic core substantially completely surrounding said coil and having a pair of field-defining edgesfacing said axis; and a plurality of hollow metal cylinders spaced along said axis to form a column for said microscope, said edges lying between said source and said cylinders, each of said cylinders having a frusto-conical axial bore therethrough with the smaller end of said bore facing said source and wi h the apex of the conical angle of said bore located between the center of the longitudinal gap bounded by said edges and the proximal end of the respective one of said cylinders.

'2. An electron microscope comprising: a' source producing a beam of electrons traveling along an axis; an electromagnetic lens comprising a coil of Wire substantially coaxial with said beam and a hollow toroidal ferro: magnetic core substantially completely surrounding said coil and having a pair of field-defining edges longitudinally spaced apart and facing said axis to' define the location of said lens on said axis, a portion of the inner bore of said core being cylindrical; and a plurality ofhollow ferromagnetic cylinders supported within said cylindrical portion of said bore and spaced along said axis with said edges being located between said cylinders andsaid source, each of said cylinders having a frusto-conical axial bore therethrough with the smaller end of said axial bore facing said source and with the apex of the conical angle of said axial bore being located between the center of said electron lens and the proximal end of the respective one of said cylinders.

3. The device of claim 2 comprising, in. addition, a hollow cylindrical tube having an outer diameter fitting snugly within the cylindrical portion of the bore of said core and an inner diameter fitting snugly around said cylinders.

Y 4. An electron microscope comprising: a source producing a beam of electrons traveling along an axis; a plurality of electromagnetic lens structures spaced along said axis, each of said lens structures comprising a coil of wire substantially coaxial with said beam and a ferromagnetic core partially enclosing the respective coil, each of said ferromagnetic cores comprising an annular disc coaxial with said axis and a tubular portion forming a longitudinal extension of the periphery of each of said discs; a column comprising a plurality of hollow, tubular members extending through said coils and along said axis; a plurality of vacuum-tight seals between said tubular members and said annular discs; and a plurality of ferromagnetic fiux-concentrating structures. within said col-.

umn, at least one of said flux-concentrating structures forming part of an electron lens and having a cylindrical bore extending part way therethrough; a non-magnetic tubular spacer fitting snugly within said bore; and a plurality of hollow ferromagnetic cylinders fitting snugly within said tubular spacer, each of said cylinders having a conical, axial bore therethrough with the smaller end of said bore facing said source and with the apex of the included conical angle of said bore located between the center of said electron lens on said axis and the proximal end of the respective one of said cylinders.

' References Cited in the file of this patent UNITED STATES PATENTS 

1. AN ELECTRON MICROSCOPE COMPRISING: A SOURCE PRODUCING A BEAM OF ELECTRONS TRAVELING ALONG AN AXIS; AN ELECTROMAGNETIC LENS STRUCTURE COMPRISING A COIL OF WIRE SUBSTANTIALLY COAXIAL WITH SAID BEAM AND A HOLLOW TOROIDAL FERROMAGNETIC CORE SUBSTANTIALLY COMPLETELY SURROUNDING SAID COIL AND HAVING A PAIR OF FIELD-DEFINING EDGES FACING SAID AXIS; AND A PLURALITY OF HOLLOW METAL CYLINDERS SPACED ALONG SAID AXIS TO FORM A COLUMN FOR SAID MICROSCOPE, SAID EDGES LYING BETWEEN SAID SOURCE AND SAID CYLINDERS, EACH OF SAID CYLINDERS HAVING A FRUSTO-CONICAL AXIAL BORE 